Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like. As such, a number of methods may be used to control the transmission to improve idle-stops and subsequent restarts, when restart conditions are met.
One such example is illustrated by Suzuki et al. in U.S. Pat. No. 6,556,910 B2. Therein, a plurality of transmission forward clutches are controlled by a hydraulic servo to shift the clutches between engaged and disengaged states when adjusting between idle-stop and restart conditions. Specifically, when an idle-stop condition is satisfied, the transmission is maintained in gear and a hydraulic pressure of the hydraulic servo is also maintained at a predetermined pressure.
However, the inventors have recognized several potential issues with such a method. As one example, during idle-stop conditions, the time required to stop the engine, for example the time required to drop the engine speed from 700 RPM to zero, may be longer than desired. As such, if the time needed for engine shut-down is too long, a vehicle operator may choose to restart and/or launch the vehicle before the engine speed has dropped to zero.
Thus in one example, some of the above issues may be addressed by a method of controlling a vehicle system including an engine that may be selectively shut down, the system further including a torque converter and a torque converter lock-up clutch. One example embodiment comprises, during an idle-stop engine shut-down, restricting flow of transmission fluid out of the torque converter, and adjusting engagement of the torque converter lock-up clutch to adjust a drag torque on the engine to stop the engine.
In one example, a flow restriction valve may be included in the hydraulic circuit of a torque converter to thereby restrict (for example, fully restrict or partially restrict) the flow of transmission fluid out of the converter. As such, restricting flow out of the torque converter may include restricting a flow of transmission fluid into a system cooler and/or lube. The position of the flow restriction valve may be varied based on the nature of the torque converter. For example, when the torque converter is a two-pass torque converter, the flow restriction valve may be positioned in a converter release circuit. Alternatively, when the torque converter is a three-pass or closed-piston type torque converter, the flow restriction valve may be positioned in a clutch out circuit. During an idle shut-down operation, that is, when the engine is shut-down responsive to idle-stop conditions and without receiving a shut-down request from the vehicle operator, an engine controller may close the flow restriction valve to enable an increase in the torque capacity of the torque converter lock-up clutch. As such, during an engine idle-stop shut-down, the drop in engine speed leads to a corresponding drop in output from an engine-driven mechanical pump. The consequent drop in hydraulic pressure may reduce the capacity of transmission clutches, such as the torque converter lock-up clutch. Herein, by closing the flow restriction valve during the engine shut-down, at least some clutch capacity may be restored (for example, less than full capacity may be restored), by restoring at least some hydraulic pressure.
To expedite engine shut-down, the controller may further command the torque converter to be locked up, by adjusting an engagement of the torque converter lock-up clutch, to thereby apply a drag torque on the engine to stop the engine. A degree of engagement of the torque converter lock-up clutch may be adjusted responsive to operating conditions, such as an engine speed and/or a desired stopping position of the engine. In one example, when the engine speed is above a desired engine speed, the engagement of the torque converter lock-up clutch may be increased to increase the drag torque applied. In another example, when the engine speed is below a desired engine speed, the engagement of the torque converter lock-up clutch may be decreased to decrease the drag torque applied on the engine. In one example, the increased engagement of the torque converter lock-up clutch and the closed position of the flow restriction valve may be maintained until shut-down has been completed. Then, after completing the shut-down, the flow restriction valve may be opened to un-restrict flow of transmission fluid out of the torque converter, the engagement of the torque converter lock-up clutch may be reduced (for example, the lock-up clutch may be disengaged), and the torque converter may be unlocked. During a subsequent engine restart, the engine may be cranked with the torque converter lock-up clutch in the reduced engagement condition (or disengaged condition) and the flow restriction valve open. Then, as the engine speed rises, with the flow restriction valve open, the engagement of one or more transmission clutches may be modulated (for example, the engagement of a forward clutch may be increased). The flow restriction valve may also enable improved pressure control during conditions when the transmission fluid has lower flow rates through the torque converter.
In this way, a torque converter lock-up clutch may be advantageously used to apply a drag torque and expedite engine shutdown even during conditions of reduced pump output to the clutches. Specifically, by using a flow restriction valve to reduce flow of transmission fluid out of the torque converter, hydraulic pressure and torque-converter clutch capacity may be maintained even during conditions of reduced pump output. Furthermore, a duration of torque converter lock-up during engine shut-down conditions may be increased (for example, by increasing a duration of torque converter lock-up clutch engagement). By adjusting a degree and/or duration of engagement of a torque converter lock-up clutch, a drag torque may be applied to counteract a rotation of the engine by the ground, through the wheels and/or powertrain, thereby providing a faster engine shut-down. In addition to enabling a faster engine shut-down, crankshaft oscillations due to cylinder air-spring effects after the engine speed had reached zero, may be significantly dampened. In this way, repeated stop/start events may be better supported.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.